In ink-jet printing methods, droplets of ink are directly projected onto a printing medium from very fine nozzles and allowed to adhere to the printing medium to obtain printed materials on which characters or images are printed. The ink-jet printing methods need no printing plate unlike the conventional printing methods. Therefore, it has been expected that the ink-jet printing methods can be used in extensive applications as on-demand printing methods that are adaptable for production of a small number and various kinds of prints. In particular, in recent years, there is an increasing demand for not only printed materials using a conventional printing medium having a white ground, such as a plain paper, but also printed materials using a printing medium having a non-white ground, such as a corrugated board, a paper board, a resin film, etc.
When producing printed materials using the printing medium having a non-white ground, a white ink is used for the purpose of expressing images with a white color or enhancing visibility of the images. As a pigment of the white ink, titanium oxide that is an inorganic pigment having a high hiding power has been frequently used.
In addition, for the purpose of improving dispersibility of the titanium oxide in the ink, a pigment dispersant has been used.
For example, JP 60-123564A (Patent Literature 1) discloses an aqueous pigment dispersion constituted of a polymer dispersant, a pigment such as titanium oxide and an aqueous medium, in which the polymer dispersant serves as a pigment dispersant that is capable of easily dispersing the pigment in an aqueous coating material even when using the dispersant in a small amount and causes no deterioration in coating performance of the aqueous coating material, and is produced by copolymerizing monomers including 3 to 98 parts by weight of a mono- or polyalkylene glycol-modified (meth)acrylic acid-based monomer, 2 to 97 parts by weight of an ethylenically unsaturated nitrogen-containing monomer and 0 to 20 parts by weight of an ethylenically unsaturated carboxylic acid.
JP 2009-24165A (Patent Literature 2) discloses a pigment dispersant that is constituted of 5 to 30% by mass of an aromatic or heterocyclic vinyl monomer unit, 10 to 30% by mass of an acid group-containing monomer unit, 40 to 80% by mass of a (meth)acrylic acid ester monomer unit and 5 to 30% by mass of a monomer unit containing a polyalkylene glycol chain having a specific molecular weight or a chain of a monoalkyl ether of the polyalkylene glycol, and has an acid value of 30 to 300 mgKOH/g and a number-average molecular weight of 5,000 to 30,000. In the Patent Literature 2, it is described that titanium oxide is used as a pigment, and a pigment dispersion using the pigment dispersant can be used as a colorant for an aqueous coating material, an aqueous gravure ink, an aqueous ink-jet ink, an aqueous ink for stationeries, etc.
On the other hand, an ink-jet printing apparatus having a mechanism of inhibiting precipitation of titanium oxide in an ink has been developed. For example, JP 2011-121344A (Patent Literature 3) discloses an ink-jet printing apparatus that is equipped with a main tank, a sub tank, an ink supply means and an ink recovery means for the purpose of inhibiting precipitation of the titanium oxide in a white ink containing the titanium oxide as a pigment even after allowing the white ink to stand in the ink tank for a long period of time, and it is also described that by circulating the ink that is present in an ink flow path between the ink supply means and the ink recovery means, it is possible to maintain a concentration of the ink at a constant level.